Quenching of X-ray emission in little red dots by both Compton-thick gas and high accretion rates

Abstract

Little red dots (LRDs) are candidate high-redshift supermassive black holes accreting in dense gas. They remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the \sirocco radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use these constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray spectral energy distribution, and observed bandpass as a function of redshift. The X-ray constraints are very tight, requiring extinction by a Compton-thick gas column ($N_{\mathrm{H}}\sim 10^{25}{\mathrm{cm}}^{-2}$) with moderate metallicity $0.05$-$0.1Z_{\odot }$ and intrinsically weak X-ray emission (bolometric to X-ray luminosity ratio, $k_{\mathrm{bol},\mathrm{X}}\gtrsim 30$), as observed in narrow-line active galactic nuclei with high accretion rates, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line active galactic nuclei would be detected even behind the typical Compton-thick gas columns with modest metallicity that were inferred from the optical spectra. Very low metallicity objects might be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.

Accepted for publication in A&A, 7 pages, 5 figures